OpenGL does not render to screen by calling glBindFramebuffer(GL_FRAMEBUFFER,0) - c++

Question

Normally this code renders the image of the texture to screen. But if I now add the command glBindFramebuffer(GL_FRAMEBUFFER,0) to the code, it will not render anything. It renders the screens just in the color of glClearColor. I am working with QT so using QOpenGLWidget.
glViewport(0, 0, _width, _height);
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glClearColor(0.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT|GL_STENCIL_BUFFER_BIT);
glDisable(GL_DEPTH_TEST);
_program.bind();
glBindVertexArray(_vao);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, textureID);
glUniform1i(glGetUniformLocation(_program.programId(), "u_texture"), 1);
glDrawArrays(GL_TRIANGLES, 0, 6);
glBindVertexArray(0);
Want can go possibly wrong?
EDIT:
/* final.fsh */
# version 330 core
uniform sampler2D u_texture;
in vec4 qt_TexCoord0;
out vec4 fragColor;
void main(void)
{
fragColor = texture(u_texture, qt_TexCoord0.xy);
}
/* final.vsh */
# version 330 core
layout (location = 0) in vec3 a_position;
out vec4 qt_TexCoord0;
void main(void)
{
gl_Position = vec4(a_position, 1.0);
const mat4 B = mat4(0.5, 0.0, 0.0, 0.0,
0.0, 0.5, 0.0, 0.0,
0.0, 0.0, 0.5, 0.0,
0.5, 0.5, 0.5, 1.0);
qt_TexCoord0 = B * vec4(a_position, 1.0);
}
/* Loads VAO to GPU */
GLfloat max_ = 1.0;
GLfloat min_ = -1.0;
GLfloat vert[] = {
max_, max_, 0,
max_, min_, 0,
min_, max_, 0,
max_, min_, 0,
min_, min_, 0,
min_, max_, 0,
};
glGenVertexArrays(1, &_vao);
glBindVertexArray(_vao);
GLuint vbo;
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vert), vert, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, sizeof(GLfloat)*3, (GLvoid*)0);
glBindVertexArray(0);

Answer 1

As stated by G.M., and according to the docs, glBindFramebuffer(GL_FRAMEBUFFER, 0) "breaks the existing binding of a framebuffer object to the target". You need this call after having used your framebuffers to do some offscreen rendering. When you want to finally render to the screen, you have to render the the back buffer, by calling glDrawBuffer(GL_BACK).
So your render loop might look like
// Do some rendering stuff in n offscreen textures
glBindFramebuffer(GL_FRAMEBUFFER, the_buffer);
glDrawBuffers(n, buffers);
// Rendering calls
// Finally use the results for the final rendering
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glDrawBuffer(GL_BACK);
// Rendering calls
EDIT:
It appears you are using QOpenGLWidget as your OpenGL provider / windowing system. The Qt guys are using framebuffers to render their UI, and the content of the result of your QOpenGLWidget is drawn inside an FBO.
So, what it means (at least in my experience, there might be a better way to tackle this problem), is that you cannot render in the backbuffer anymore, you have to render in Qts framebuffer.
One way to do this (again, it might not be the best way at all, but it has worked for me), is to save the bound framebuffer at the begining of your rendering, and do your final rendering in this buffer. Which leads to
// First of all, save the already bound framebuffer
GLint qt_buffer;
glGetIntegerv(GL_FRAMEBUFFER_BINDING, &qt_buffer);
// Do some rendering stuff in n offscreen textures
glBindFramebuffer(GL_FRAMEBUFFER, your_buffer);
glDrawBuffers(n, buffers);
// Rendering calls
// Finally use the results for the final rendering
if (glIsFramebuffer(qt_buffer))
{
glBindFramebuffer(GL_FRAMEBUFFER, 0);
glDrawBuffer(GL_BACK);
}
else
{
glBindFramebuffer(GL_FRAMEBUFFER, qt_buffer);
glDrawBuffers(1, buffers);
}
// Rendering calls

Answer 2

I have the same issue and finally I find the cause is glBindFramebuffer(GL_FRAMEBUFFER, 0);
QOpenGLWidget's default framebuffer may not be 0, so you need to get it by calling QOpenGLContext::defaultFramebufferObject()
so change like this:
glViewport(0, 0, _width, _height);
glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebufferObject()); //<-- change here
glClearColor(0.0,0.0,0.0,1.0);
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT|GL_STENCIL_BUFFER_BIT);
glDisable(GL_DEPTH_TEST);
_program.bind();
glBindVertexArray(_vao);
glActiveTexture(GL_TEXTURE1);
glBindTexture(GL_TEXTURE_2D, textureID);
glUniform1i(glGetUniformLocation(_program.programId(), "u_texture"), 1);
glDrawArrays(GL_TRIANGLES, 0, 6);
glBindVertexArray(0);